I'll assume the C is capitalized and so is the O. There is no single element or atom with a A, so I'll automatically assume that a is with the C, therefore making the element Ca (Calcium), and O as single is Oxygen.
Normally, O is diatomic (it has the subscript 2) but since it's a compound (bonded with another), it's already bonded with something so it doesn't need to be diatomic (does not need the subscript 2).
Calcium + Oxygen = Calcium Oxide = CaO
Remember that you need to capitalize the first letter of the start of each element. The capitalization matters because it tells us the start of which element and which and how many elements are present if any.